Hello all, I own a 2002 Corvette with 58k miles.

I recently noticed my long term fuel trim was running very lean at 15% at idle only. My short term fuel trim is fluctuating normally. When at idle, I give the car gas and the lean condition fixes itself, highly indicating a vacuum leak.

Anyways I think I found out the problem. I have a vacuum leak, behind the intake manifold, right below the MAP sensor, there is a nipple for a vacuum hose for the HVAC vent actuator thing. Well, there was nothing connected to that vacuum nipple, and I found the piece of vacuum line hanging below. I am pretty sure this is the reason my car is running lean.

I also have a problem that is most certainly related to my first issue. My A/C blows out of all vents and I can not change the direction of the air flow. I have read through the forums that this is one of the most common problem with that vacuum hose behind the intake that gets old and deteriorates.

I just want to confirm with you C5 experts if this is the most likely reason my car is running lean. I will be removing the intake manifold tmrw morning to replace the hose that runs from the back of the intake, down to the passenger fender to the vacuum reservoir tank.

Thanks.

 

Yes. I had a vacuum issue and it was the same.

 

Yes !!...positive fuel trims that decrease with an increase at RPM indicates a vacuum leak.

 

I'm glad both of you agree, I am almost certain this is the issue. I will update the thread tmrw to see if reconnecting that unplugged vacuum line fixes my lean condition.

 

Let us know...a good 2 part video to help with your understanding of fuel trims !!

 

Wow! Thank you! I believe I have already watched these two videos but I will re-watch them as it has been a while.

 

Just got done finishing the job!

Good news, my A/C vent control now works! I can choose where to focus the air!

Sorta bad news, I am still running lean but at a LTFT of 9%. Better than the 15% though! I didn't let the car get to operating temperature as it was very late. So maybe it'll fix itself tmrw when I take it out for a good drive.

Should I disconnect the battery to make the computer relearn the fuel trims?

 

It takes a while for those Long Terms to decrease...try taking the car for a spin and you should be OK !!

 

Took it out for a good drive and my long term was +10.2%. Now it's around +6% to +9% fluctuating.

Giving it gas drops the long term to 0% immediately. Could still be a vacuum leak somewhere, or it could be the cam the car has.

Who knows, all I know is that my car has better gas mileage and blows super hard after fixing that enormous vacuum leak.

I'll update the thread to follow up on my LTFT's journey to hopefully close to or around 0% to +/-5%

 

Still running lean at idle with +10.2%. Lean condition fixes itself almost immediately upon accelerating the car, almost certain it is a vacuum leak. Gonna go buy a 99 cent cigar and give the car a smoke test. Local mechanic shops are charging me $100 just for a smoke test, so I am just gonna save me some cash and go with the cigar route.

We'll see what happens!

 

I would agree that you almost certainly still have a vacuum leak. Good luck finding it.

 

Moved to C5 Tech.

 

I think I have fixed it, fingers crossed! My lean condition seem to have gotten worse today running at +20% to +22% at idle (My LTFT). So I knew I had to fix the issue pronto because that was just bothering me like crazy, even though the car was not throwing any codes and was not running any different.

Turns out I had some loose connections on the purge valve (I think that is what it is called). It is located on the drivers side, right next to the injectors. I messed with both connections that go to the purge valve and surprisingly my LTFT started to get better and went from +20% all the way down to +5.5%, and my STFT is amazingly at 0%!

I think I am going to leave it at that for now, I can live with +5.5% LTFT. I do believe I have a small vacuum leak somewhere because giving throttle drops the LTFT to +3%.

One day I will find the vacuum leak or whatever it could be. For now I think I have solved my issue. I will still continue to update the thread if my car begins running lean again, but let's hope not!

Thank you all.

 

Knowing how the purge valve operates electrically I don’t see how loose connections would cause the fuel trims to decrease...the purge valve is normally closed but will open when the PCM does it EVAP test !!

 

I did not know that, I also was messing with other vacuum lines around the engine bay but none were as loose as those lines on the purge valve.

 

You were right! That purge valve had nothing to do with it. My lean condition returned sadly and it is a weird situation. Sometimes the LTFT will read 0% to +1.2% which is awesome... then while driving the car around town and coming to a stop my LTFT shoots up to +10% to +18% at idle! I have no idea what is going on, I cleaned out the MAF sensor with MAF cleaner, I checked my O2 sensors for any weird readings but they all checked out.

This issue is starting to become an intermittent issue, my guess would be some sensor is starting to go out with a combination of a small vacuum leak.

Further information, ZERO codes. Not sure when the PCM throws a P0171 code but I haven't gotten one yet. Also I do not have a high idle nor any performance issues. My car has aftermarket header, so the PO did not remove the entire air pump assembly and left the part that runs under the drivers side headlight. Seems they removed the part that goes behind the intake, not sure if this could cause vacuum issues?

Kind of at a loss, I can drive the car fine and all but it just bothers me it runs lean. Throw me ideas! Soon I am going to check the fuel pressure once I rent a gauge from autozone.
Any advice would be appreciated

 

When coming to a stop if the fuel trims increase that signifies a vacuum leak. The PCM will throw the P0171 when the long term hits 23%. You may just need to take it to a shop that has a smoke machine to see where and if you have a vacuum leak !!

 

I made my own smoke machine at home and it works surprisingly well! Throws a bunch of smoke and it really helped narrow the situation.

Here are the results, I did not see smoke come out of anyone from the top or back of the engine. The only place I saw a good amount of smoke was in front of the engine, more specifically above the harmonic balancer.

Is there a vacuum line in front of the engine above the harmonic balancer area???

 

Took my car to a shop to get it smoked and turns out the car has zero vacuum leaks supposedly. Also the problem is getting worse as it now capped at +25% at idle. Part throttle and WOT fuel trims are perfect.

A member said that a code will be thrown at +23%, well the car has zero codes. So I don't know if it is possible someone tuned out the code or what. I am starting to guess someone did not tune the car correctly as it has a CAI, LT headers, and a camshaft which can cause lean conditions.

I'm out of ideas, what do you all think? MAF sensor?

 

On my previous car (1999 Mazda Miata) I had a similar lean condition issue, similar to what I have going on right now. I fixed the issue on my Miata by replacing the MAF sensor.

So after lots of thinking, I thought it was time I took the chance on replacing the MAF sensor on my Corvette. Well to my surprise I think that was the issue. I bought a new MAF sensor from Autozone for $92, eesh. RockAuto has a MAF sensor for $40, which I am going to order then replace with the Autozone MAF sensor to later return it and save me $50 (Don't tell Autozone).

Anyways upon installing the new MAF sensor my fuel trims were adjusting right away and my LTFT finally settled at +2.2% (Occasionally goes to +7%). I took it out for a long drive and the LTFT never saw past +10%. I think it is safe to say I got this one solved. Weird that it seemed like a vacuum issue... but at least the issue at hand was fixed.

Hopefully this thread helps anyone battling a weird lean condition issue that presents itself as a vacuum leak which in this instance was never the case.